1. Field of the Invention
This invention relates to apparatus and methods for image data compression.
2. Description of the Prior Art
In an image data compression system, often the output bit rate of a compressed data stream is constrained by the capacity of a transmission or storage channel. A particular example of this is when compressed video data has to be recorded on a tape channel; generally, to maintain real-time operation, a whole field or frame's worth of compressed data has to be recorded onto a predetermined number of record tracks.
In order to comply with this constraint, the bit rate of the output compressed data is controlled by controlling the degree of compression applied. In many cases, this is achieved through controlling a quantisation factor applied to the video data (e.g. after transformation to the frequency domain), before the quantised data is subjected to entropy encoding or the like.
The effect of varying the quantisation factor, followed by the entropy encoding process, on the quantity of compressed data is highly non-linear and not easily predictable. While this process generally leads to an output bit rate less than or equal to the available bit rate, the unpredictable nature of image data compression can mean that sometimes the available bit rate is exceeded. There is then the problem of discarding some of the compressed data in a way that endeavours to avoid producing image artifacts that are visible and subjectively disturbing to the user.
It is an object of the invention to provide an improved data compression technique.